Coastal Circulation

1] The characteristics of a persistent gyre in the mouth of the Bay of Fundy are studied using model simulations. A set of climatological runs are conducted to evaluate the relative importance of the different forcing mechanisms affecting the gyre. The main mechanisms are tidal rectification and density-driven circulation. Stronger circulation of the gyre occurs during the later part of the stratified season (July-August and September -October). The density-driven flow around the gyre is set up by weak tidal mixing in the deep basin in the central Bay of Fundy and strong tidal mixing on the shallow flanks around Grand Manan Island and western Nova Scotia. Spring river discharge has an important influence on near-surface circulation but only a small effect when averaged over the entire water column. Retention of particles in the gyre is controlled by the residual tidal circulation, increased frontal retention during stratified periods, wind stress, and interactions with the adjacent circulation of the Gulf of Maine. Residence times longer than 30 days are predicted for particles released in the proximity of the gyre.

1] A persistent gyre at the mouth of the Bay of Fundy results from a combination of tidal rectification and buoyancy forcing. Here we assess recent interannual variability in the strength of the gyre using data assimilative model simulations. Realistic hindcast representations of the gyre are considered during cruises in 2005, 2006, and 2007. Assimilation of shipboard and moored acoustic Doppler current profiler velocities is used to improve the skill of the simulations, as quantified by comparison with nonassimilated drifter trajectories. Our hindcasts suggest a weakening of the gyre system during May 2005. Retention of simulated passive particles in the gyre during that period was highly reduced. A recovery of the dense water pool in the deep part of the basin by June 2006 resulted in a return to particle retention characteristics similar to climatology. Retention estimates reached a maximum during May 2007 (subsurface) and June-July 2007 (near surface). Interannual variability in the strength of the gyre was primarily modulated by the stratification of the dense water pool inside the Grand Manan Basin. These changes in stratification were associated with mixing conditions the preceding fall-winter and/or advectively driven modification of water mass properties.

A high-resolution, 3-dimensional primitive equation model is used to investigate the cross-shelf exchange in the East China Sea (ECS). Favorable comparisons between field data and model simulations from both climatological run and hindcast run for 2006 indicate that the model has essential skills in capturing the key physics of the ECS. Temporal and spatial variations of the cross-shelf exchanges are further analyzed. It was demonstrated from both observations and simulations that in 2006 high saline water could be delivered to the north of the Changjiang River mouth (near 32ºN) as a result of stronger than typical cross-shelf exchanges at the shelf break and flows through the Taiwan Strait with an annual mean rate of 2.59 Sv and 1.83 Sv, respectively. A few new places at the shelf break were also identified where persistent and vigorous onshore or offshore exchanges occur throughout the year. Cross-shelf exchange is largely determined by the along-shelf geostrophic balance with weak seasonality, which is modulated in upper layers by northeasterly monsoon from early-fall to late-spring and at seabed by bottom friction during December − January, May and August − September. Nonlinear effect, with strong spatial variations and intra-seasonal variability, is a secondary but persistent contributor to the net seaward transport, except for northeast of Taiwan where the nonlinear effect becomes significant but more varied.

1] Unusually cold seawater temperatures were observed along much of the U.S. eastern seaboard during the summer of 2003. In this study, hydrographic and atmospheric observations from spring through summer were analyzed to track the evolution of the cold water event in the South Atlantic Bight (SAB) and investigate links to various forcing mechanisms. The hydrographic observations included 13 cross-shelf transects over the central region of the SAB, surface temperature time series from several NDBC stations, and bottom temperatures from a mid shelf mooring. Atmospheric data were obtained from NDBC stations. Additional data included water level from NOS stations and river discharge from USGS stations. The conditions observed during spring and summer of 2003 were compared with climatological values. Record precipitation and increased river discharge during spring produced strong salinity stratification over the inner and mid shelf. Anomalously intense and persistent upwelling-favorable winds were present from May until August. On the mid and outer shelf the resulting upwelling and subsurface shoreward penetration of cold water acted as a feedback mechanism to preserve the stratified conditions through the summer. The characteristics of the upwelled water corresponded to water from the lower part of the Gulf Stream water column. On the shelf the resulting temperature values under the thermocline were significantly lower than climatological temperatures by 5°-7°C.

The inner shelf circulation on the Abrolhos Bank is investigated using four years (2002)(2003)(2004)(2005) of moored current and bottom pressure observations from two sites in conjunction with wind data from a nearby meteorological station. This is one of the longest projects monitoring current and sea level along the Brazilian coast. The time variability of the local circulation and main forcings are described. For the first time, both the seasonal and the interannual variabilities are addressed, as are the impact of remote forcing. The cross-shore pressure gradient in the region is mostly set up by along-shore winds, whereas the sub-inertial cross-shore momentum balance is essentially geostrophic, with smaller contributions from the cross-shore wind stress. The along-shelf momentum balance is ageostrophic and mainly occurs between the wind and bottom stresses. South-southwestward along-shore currents occur between October and January, whereas stronger north-northeastward currents are observed in fall and winter. This seasonal cycle is driven by the N-S migration of the South Atlantic High between the seasons. An increasing frequency of the southern winds and, consequently, northward currents are observed between 2002 and 2005 and are related to both the number of fronts reaching the region and the remote effect of fronts that did not cross the area. The cross-shore circulation is weak and mainly forced by the tides. It is suggested that long-period shelf waves that propagate into the region change the inner shelf current field and sea level.

We examine the circulation over the inner-shelf of the Catalan Sea using observations of currents obtained from three Acoustic Doppler Current Profilers (two at 24 m and
one at 50 m) during March–April 2011. The along-shelf current fluctuations during that period are mainly controlled by local wind stress on short time scales and by remote pressure gradients on synoptic time scales. Different forcing mechanisms are involved in the along-shelf momentum balance. During storm conditions, wind stress, sea level gradients and the nonlinear terms dominate the balance. During weak wind conditions, the momentum balance is controlled by the pressure gradient, while during periods
of moderate wind in the presence of considerable stratification, the balance is established between the Coriolis and wind stress terms. Vertical variations of velocity are affected by the strong observed density gradient. The increased vertical shear is accompanied by the development of stratified conditions due to local heating when the wind is not able to counteract (and break) stratification. The occasional influence of the Besòs River plume is observed in time scales of hours to days in a limited area in near the city of Barcelona. The area affected by the plume depends on the vertical extent of the fresher layer, the fast river discharge peak, and the relaxation of cross-shore velocities after northeast storm events. This contribution provides a first interpretation of the inner-shelf dynamics in the Catalan Sea.